The present disclosure relates to ferroelectric materials and devices. In particular, the present disclosure relates to methods of electrically driving ferroelectric devices and elements.
Some ferroelectric materials exist as perovskite metal-oxide compound ceramics with a general chemical formula ABO3, where A and B are different cations. These materials crystallize in a cubic structure shown in FIG. 1A above their Curie Temperature. As shown in FIG. 1A, the ‘A’ atom sits at cube corner positions (0, 0, 0), type ‘B’ atom sits at body centre position (½, ½, ½) and oxygen atoms sit at face-centered positions (½, ½, 0).
As noted above, such centrosymmetric (cubic) structures exist only at high temperatures (above Curie temperature). At temperatures below the Curie temperature, the structure transforms into a tetragonal form, as shown in FIG. 1B. The distinct feature of the tetragonal structure is the presence of a non-zero polarization due to a shift of atom “B” from its centrosymmetric position.
The tetragonal phase also exhibits pyroelectric and ferroelectric properties, such that the crystal domains possess a spontaneous polarization in the absence of an external electric field. In ferroelectrics, the polarization direction can be reversed under the application of a sufficiently large external electric field. Such piezoelectric crystals change their macroscopic dimensions in response to an external electric field. This is the property that is utilized in ultrasound transducers and generators, and other devices. In particular, lead-based perovskites PbZrxTi1-xO3 (PZT) have emerged as one of the most widely studied and technologically important class of ferroelectric oxides. This alloy exhibits an enhancement of electromechanical response near to the morphotropic phase boundary (MPB) at x≈0.4-0.5 that exceeds by far the properties of individual constituents PbZrO3 and PbTiO3. The enhancement of the piezoelectric response near MPB is attributed to “flattening” of an energy surface that facilitates inversion of the spontaneous polarization [1-4].
FIG. 2A shows a typical piezoelectric element 100, consisting of a ferroelectric piezoelectric material sandwiched between two contacts 105 and 110, as shown in FIGS. 2A and 2B. When an alternating potential is applied to the contacts, the crystal undergoes expansion/contraction cycles, due to the electric field 120.
During the cycle, the central atom switches its position, as shown in FIG. 3, which results in a polarization inversion [1]. For the switching process to occur, the electric field (or bias voltage) must exceed its critical (coercive) magnitude, which is a material-specific property. The coercive field Ec, is the electric field at which the polarization inversion occurs, as shown in the figure.
The notable feature of the polarization vs. applied electric field (or bias voltage) plot is the existence of a hysteresis loop (see FIG. 3). The area of this loop determines parasitic losses in the ferroelectric crystal, which are responsible for incomplete conversion of the electrical power into useful mechanical signal. The unused electrical energy can partly transform into the heat, similar to the dielectric heating in a microwave oven. Heating of the piezoelectric element is an unwanted effect, and requires special care for its dissipation [2]. In practical applications, this limits the functionality of certain devices by causing the element to overheat. Therefore, a reduction of the coercive electric field (or bias voltage) is important for improvement in the efficiency of piezoelectric actuators and reduction of the electrical power required to drive the transducer.